Use of Organometallic Compounds as Catalysts in Chemistry and Industries

1. C ATA LY S I S B Y O R G A N O M E TA L L I C
C O M P O U N D S
P A P E R I I – O R G A N O M E T A L L I C S & M A I N
G R O U P C H E M I S T R Y
- J A I S WA L P R I YA N K A
- M . S C . I I
( I N O R G A N I C )
- S E M E S T E R I V
- M I T H I B A I C O L L E G E
( 2 0 1 5 - 1 6 )

2.  CATALYSIS BY ORGANOMETALLIC
COMPOUNDS
• A catalyst is a substance that increases the rate of a reaction but it is not itself
consumed.
• Catalysis plays a vital role in the production of fuels, commodity chemicals, fine
chemicals and pharmaceuticals as well as providing the means for
experimental safeguards all over the world.
• More than 60% of all chemical products and 90% of all chemical processes are
based on catalysis.
• Most catalysts used in industrial and research laboratories are inorganic (often
organometallic) compounds.

3.  CATALYSIS (CONTD)…..
• A whole new technology appeared based on organometallic catalysis in
olefin polymerization.
• Nobel prizes for chemistry have been awarded to Zieglar and Natta
(1963), Fischer and Wilkinson (1973) for their discoveries in
Organometallic chemistry and homogeneous catalysis.
• More recently, in 2005, Chauvin, Schrock, and Grubbs were awarded
Nobel Prize for developing organometallic catalysts for olefin
methathesis.
• Catalysis can be of two types Heterogeneous and Homogeneous.

4. Types of Catalysts
1) Homogeneous : Catalyst and Reactant in the same phase
e.g.) Organometallic Compounds
2) Heterogeneous : Catalyst and Reactant in different phases
e.g.) Metal Surface
Cativa Process
Catalytic decomposition of formic acid on noble metals

5.  Homogenous versus Heterogenous Catalysis
Homogenous
(Monophase)
Heterogenous
(Biphase/Multiphase)
Form Soluble metal complexes Metals, usually
supported or oxide
Temperature Low (<250OC) High (250-500OC)
Activity (of metal
content)
High Variable
Selectivity High Variable
Conditions of Reaction Mild Harsh
Lifetime of Catalysts Variable Long
Sensitivity of
Deactivation
Low High
Problems due to
Diffusion
None Difficult to solve
Recycling of Catalysts Usually difficult Can easily be done
Mechanism Realistic models do exist Not obvious

6.  Heterogeneous Catalysis
Desorption of Products (STEP 4)
There is a re-arrangement of electrons and the products are then released from the active sites
Adsorption of Reactants (STEP 1)
Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the
bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.
Reaction of adsorbed intermediates (STEPS 2 and 3)
Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur.
This increases the chances of favourable collisions taking place.
Adsorptive property of a catalyst = Catalytic activity

7.  Homogeneous Catalysis
• Catalytic steps in homogeneous reactions
1. Association / dissociation of a ligand (requires labile complexes)
Catalysis steps often requires facile coordination of reactants to metal ions and equally facile loss of
products. Both processes must occur with low Activation Energy. For this purpose, highly labile
complexes are needed as they are co-ordinatively unsaturated (having an open coordination site or
being weakly coordinated)
e.g. Square-planar 16-electron complexes are co-ordinatively unsaturated.
ML4 complexes of Pd(II), Pt(II) and Rh(I) - [RhCl(PPh3)3] – hydrogenation catalyst
2. Insertion and elimination reactions
The migration of alkyl and hydride ligands to unsaturated ligands (Migratory insertion)

8. 3. Nucleophilic attack on a coordinated ligand
The coordination of ligands (CO, alkenes) to metals in positive oxidation states results in the
activation of coordinated C atoms towards attack by nucleophile.
e.g. Stereochemical evidence indicates that the reaction occurs by direct attack on the most
highly substituted C atom of the coordinated olefin.
4. Oxidative addition / reductive elimination
Oxidative addition of a molecule AX to a complex brings about dissociation of the A – X bond
and coordination of the two fragments.
Reductive elimination is the reverse of oxidative addition and often follows it in a catalytic

9.  Wilkinson’s Catalyst
 RhCl(PPh3)3 was the first highly active homogeneous
hydrogenation catalyst and was discovered by Geoffrey
Wilkinson (Nobel prize winner for Ferrocene) in 1964.
 Wilkinson’s Catalyst is a Rh(I) complex, Rh(PPh3)3Cl
containing three phosphine ligands and one chlorine.
 As a result of the olefin insertion (hydrogen migration) we
obtain a Rh (III), 16e-, five coordinate species. A solvent
occupies the sixth coordination site to take it to a 18e-
species.
 Reductive elimination occurs to give the hydrogenated
product and the catalytically active species.

10. Hydrogenation mechanism
Steps:
(1) H2 addition,
(2) alkene addition,
(3) migratory insertion,
(4) reductive elimination
of the alkane,
regeneration of the
catalyst

11.  Wilkinson’s catalyst selectivity
Hydrogenation is stereoselective:
Rh preferentially binds to the least sterically hindered face of the olefin:

12.  Asymmetric hydrogenation
A variety of bidentate chiral diphosphines have been synthesized and used to make amino acids
by hydrogenation of enamides:
 One area where homogeneous catalysis rules is asymmetric hydrogenation. This involves the use
of chiral catalyst and a prochiral alkene substrate that generates a chiral carbon center in
hydrogenation.

13. Synthesis of derivative of L-dihydroxyphenylalanine

14.  Chiral hydrogenation catalysts
 Catalysts similar to Wilkinson’s but using chiral
phosphine ligands have been used for the
asymmetric hydrogenation of small molecules .
 Important in the fine chemicals /pharmaceutical
industry
 Noles and Noyori received the 2001 chemistry Nobel
prize for the development of asymmetric
hydrogenation catalysis.

15.  Intermediates in Noyori’s transfer hydrogenation

16.  Lanthanide Hydrogenation Catalysts
• Tobin Marks reported
the extraordinary
activity of (Cp2LuH)2 for
the hydrogenation of
alkenes and alkynes.
• The monometallic
complex catalyzes the
hydrogenation of 1-
hexene with a TOF =
120,000 hr-1 at 1 atm
H2, 25ºC!!
• This is one of the most
active hydrogenation
catalysts known.

17.  Ziegler-Natta Catalysis for the Polymerization of olefins
• Polymers are large molecules with molecular weights in the
range of 104 to 106. These consist of small building units known
as monomers.
For example polyethylene is made up of ethylene monomers
• In all of these cases a single monomer is repeated several times
in the polymer chain. The number of repeating units determines
the molecular weight of the polymer.

18. • The German chemist Karl Ziegler (1898-1973) discovered in
1953 that when TiCl3(s) and AlEt3 are combined together they
produced an extremely active heterogeneous catalyst for the
polymerization of ethylene at atmospheric pressure.
• Giulio Natta (1903-1979), an Italian
chemist, extended the method to
other olefins like propylene and
developed variations of the Ziegler
catalyst based on his findings on
the mechanism of the
polymerization reaction.
• The Ziegler-Natta catalyst family includes halides of titanium,
chromium, vanadium, and zirconium, typically activated by
alkyl aluminum compounds
Ziegler and Natta received the Nobel Prize in
Chemistry for their work in 1963.

19.  There are typically three parts to most
polymerizations:

21.  REFERENCES
1. Organometallic Chemistry and Catalysis, Didier Astruc
2. Organometallic Chemistry, R.C. Mehrotra
3. Inorganic Chemistry: Principles of Structure and Reactivity, James E.
Huheey, Ellen A. Keiter, Richard L. Keiter, Okhil K. Medhi
4. Reaction Mechanisms of Inorganic and Organometallic Systems,
Robert B. Jordan; Professor of Chemistry, University of Alberta
5. http://www.chem.iitb.ac.in/~rmv/ch102/ic6.pdf